Heretofore, it has been well known to provide 2.times.2 optical switches having two ports on each side, wherein the switch is configurable to make a connection between ports 1 and 2 and simultaneously to provide a connection between ports 3 and 4. Alternatively, such switches are configurable to provide simultaneous connections between ports 1 and 4, and ports 3 and 2. Hence these prior art switches have two states; a first state wherein two bar connections are formed and a second state wherein 2 cross connections are formed. It is desirable to provide an optical switch that is rugged, substantially insensitive to temperature changes within an operating range of temperature, and relatively inexpensive to manufacture. Such switches are required to be capable of switching a beam of light propagating in a waveguide, for example an optical fibre from a first similar waveguide, to a second. The core diameter of a single mode optical fibre is approximately 10 .mu.m. Providing suitable coupling in both switching states, and providing a switch that is fast enough, and tolerant of physical disturbances is a daunting task most switch manufacturers face.
A well known optical switch made by JDS Fitel Inc. that has been sold in the United States since Feb. 11, 1992 under the product number SR22xx-ONC, includes a pair of GRIN lenses 10a and 10b having a reflector or mirror 14 that can be selectively disposed therebetween as shown in FIGS. 1a and 1b. The latter figure depicts the switch in a reflecting state with the mirror positioned between the lenses 10a and 10b. Each GRIN lens has two ports offset from the optical axis (OA) of the lens. In operation, in a first state, light launched into port 1 of lens 10a couples with port 2 port on the other GRIN lens 10b, the ports being on opposite sides of a common optical axis (OA) shared by the GRIN lenses. Similarly, in the first state an optical connection is made between the other two ports 3 and 4 on lenses 10a and 10b respectively. In a second state shown in FIG. 1b, with the mirror 14 positioned between the lenses, the optical connections between ports 1 and 2, and, 3 and 4 are broken, and new optical connections is made between each pair of ports on each respective lens. Hence two connections are made, a first between ports 1 and 3, and a second connection between ports 2 and 4.
Although this switch performs its intended function, other switches have been developed by JDS Fitel Inc. that are easier to manufacture being much less sensitive to angular and/or lateral deviation of the movable optical element disposed between the GRIN lenses. For example, from a manufacturing standpoint, it is preferable to use a transmissive optical element, in which zero or an even number of internal reflections in each plane, and/or any number of refractions, are imposed on the incident light between the lenses rather than a reflective element imposing one reflection, to route, shift, or direct a beam from one port to an alternate port when the element is disposed between lenses. Thus, by providing a transmissive element such as a prism, the switch is much less sensitive to angular deviation and misalignment of the element than a switch using a reflective element such as a mirror. For example, in comparing angular sensitivity based on a 0.05 dB excess insertion loss criterion, an existing single mirror-based switch has a typical angular tolerance of 0.007 degrees; an existing prism-based switch (as in FIG. 2a) has an angular tolerance of 0.03 degrees, whereas the transmissive optical wedge-based switch described in accordance with this invention has a angular tolerance of 1.4 degrees. FIGS. 2a and 2b illustrate a 4-port 2.times.2 optical switch having 4 GRIN lenses wherein the ports are disposed along the optical axes of the lenses. In FIG. 2a light launched into port 1 of GRIN lens 20a traverses the gap between the lenses and couples into lens 20b and exits port 2. Similarly light launched into port 3 couples to port 4 in this bar-state.
With the switch selected to be in a cross-state shown in FIG. 2b, a movable prism is positioned into the gap between the four GRIN lenses. Alternatively, the prism can be rotatable such that in a bar-state it is rotated so that its sides are parallel to the end faces of the GRIN lenses 20a, 20b, 20c, and 20d wherein no deflection occurs, and in a cross-state the prism 25 is rotated into the position shown in FIG. 2b. Manufacturing a four port 2.times.2 switch such as the one shown in FIG. 2b is difficult because in a cross-state not only does port 1 have to align with port 3, but simultaneously, port 2 must align with port 4. In the instance that opposing sides of the prism are not parallel, within certain acceptable tolerances, aligning one set of ports for example, ports 1 and 4 via deflection is possible and in fact without difficulty, however simultaneously aligning the other set of ports for example ports 2 and 3 may not be possible since the orientation and location of the four GRIN lenses is fixed.
Such manufacturing difficulties are obviated by this invention since alignment of the second set of ports is not required while the transmission element is disposed between the lenses.
Configurable add drop optical circuits require one or more switches or elements providing the functionality of switching in the event that a signal is to be added or dropped to another optical signal path. The configurable add drop circuit shown in FIG. 3a allows a n-channels multiplexed signal to pass from point A to point B while providing the capability to drop one or more of the n-channels and simultaneously add a new same channel. For example a composite signal having wavelengths .lambda.a to .lambda.n is launched into the multiplexor 30 and is passed on to point B via the multiplexor 30b. If there is a requirement to drop the launched signal having a centre wavelength .lambda.a and .lambda.c and add in new signals having a centre wavelength .lambda.a and .lambda.c the switches 31a and 31c will selected to be in a cross-switching state such that ports 1 and 2 are coupled, and ports 3 and 4 are coupled. Otherwise, if neither adding nor dropping is desired, the switch will be in a bar-state. However one problem that results, is that if the switch is the type shown in FIGS. 1a and 1b, or 2a and 2b, is that add and drop ports become coupled to one another. This can have deleterious effects and is not always a desired goal. It is often preferred to isolate the add and drop ports from one another, preventing a connection between ports 3 and 2, when ports 1 and 4 are connected. Notwithstanding, a 2.times.2 switch bar state and in a cross state does not offer this type of isolation in either of its states.
The switch in accordance with this invention provides an elegant solution to this problem. Furthermore, the switch in accordance with this invention obviates the difficult requirement of ensuring that two pairs of ports are simultaneously coupled in a bar-state and in a cross-state.
It is an object of this invention to provide a relatively inexpensive and easy to manufacture switch that will serve as a 11/2.times.2 optical switch.
It is a further object of this invention to provide an add drop circuit that does not optically couple the add and drop port with one another when that node of the switch is in a pass-through mode and not adding or dropping signals.
It is yet a further object of the invention to provide a tolerant, low loss, and reliable 11/2.times.2 switch which allows a first and second port to be connected in a first state, without allowing a third and fourth port to be connected in the same state, and which allows a first and fourth port to be connected while the third and second ports are simultaneously connected.